Effect of a gravity-compensating orthosis on reaching after stroke: evaluation of the Therapy Assistant WREX.

DESIGN: Within-subjects repeated-measures design evaluating reaching with and without the Therapy Assistant Wilmington Robotic Exoskeleton (WREX). SETTING: Laboratory. PARTICIPANTS: Stroke survivors (N=10) with chronic upper-extremity hemiparesis. INTERVENTIONS: Not applicable. MAIN OUTCOME MEASURES: Arm movement kinematics (Optotrak Certus motion detection system), muscle activity for biceps, triceps, anterior deltoid, and brachioradialis muscles (bipolar surface electromyography). RESULTS: Significant improvements of reaching distance occurred for all subjects across all targets (P<.001) when using the Therapy Assistant WREX. While the self-selected peak speed of hand movement during the reach decreased significantly with the Therapy Assistant WREX (P<.001), use of the Therapy Assistant WREX led to improved quality of movement as signified by a decrease in jerk (P<.001) and a shift in the timing of the peak speed to an earlier point in the movement (P<.001). Electromyographic muscle activity analysis showed that use of the Therapy Assistant WREX led to a reduction in biceps activity across all targets during the reach (P<.05), in conjunction with a marginally significant reduction in activity of the anterior deltoid (P<.055). No changes were observed in triceps (P=.47) or brachioradialis activity (P=.28). CONCLUSIONS: By reducing requirements for shoulder activation, the Therapy Assistant WREX improved reaching performance among stroke survivors compared with free reaching, thereby potentially facilitating practice of functional tasks.

Weakness is the primary contributor to finger impairment in chronic stroke.

OBJECTIVE: To assess the relative contributions of several neurologic and biomechanic impairment mechanisms to overall finger and hand impairment in chronic hemiparetic stroke survivors. DESIGN: Repeated-measures design. SETTING: Clinical research laboratory. PARTICIPANTS: Thirty stroke survivors with chronic hemiparesis. Fifteen subjects had severe hand motor impairment and 15 had moderate impairment, as measured with the Chedoke-McMaster Stroke Assessment. INTERVENTIONS: Not applicable. MAIN OUTCOME MEASURES: The biomechanic factors stiffness and resting flexion torque, together with the neurologic factors spasticity, strength, and coactivation, were quantified by using a custom hand manipulator, a dynamometer, and electromyographic recordings. Both passive and active rotations of the metacarpophalangeal joints of the fingers were examined. RESULTS: Although subjects in the severely impaired group exhibited statistically greater passive stiffness and resting flexion torque than their moderately impaired counterparts (P<.05), the overall effect of these biomechanic changes appeared small in relation to the deficits attributable to neurologic changes such as spasticity and, especially, weakness. In fact, weakness in grip strength and isometric extension accounted for the greatest portion of the variance between the 2 groups (eta(2)=.40 and eta(2)=.23, respectively). CONCLUSIONS: Thus, deficits in hand motor control after stroke seem to derive mainly from weakness, which may be attributable to the loss of descending corticospinal pathway activation of motoneurons.

Impact of finger posture on mapping from muscle activation to joint torque.

BACKGROUND: The mapping from muscle activation to joint torque production can be difficult to determine for the multi-articular muscles of the fingers. This relationship was examined in vivo as a function of posture in the index finger. METHODS: Five healthy adults participated in an experiment in which the seven muscles of the index finger were sequentially electrically stimulated using intramuscular electrodes. Each muscle was stimulated at 12 different finger postures consisting of specified flexion of the metacarpophalangeal, proximal interphalangeal, and distal interphalangeal joints, while fingertip forces and moments were recorded. FINDINGS: Repeated measures analysis of variance revealed that joint torques resulting from the stimulation were significantly dependent upon finger posture (p < 0.05). The magnitude of the change in joint torque across postures was generally greater than 60%. This value is much larger than the difference attributable to the increase in active muscle force that occurs at longer muscle length, in accordance with the force-length curve (10-20% for the estimated length changes). In addition, the relative distribution of the joint torques generated by a given muscle activation was dependent upon finger posture for the intrinsic muscles and the long finger flexors (p < 0.05); the ratio of one joint torque to another varied with posture for these muscles, in some cases by more than 50%. INTERPRETATION: Joint torque is a product of both muscle force and the corresponding moment arm. As the change in active muscle force was limited, these data suggest that substantial changes in muscle moment arms occur with posture. Therefore, this postural dependence should be considered when constructing biomechanical models of the hand or planning tendon transfers for the fingers.